Rig model with existing armature
So in short, you have a "deform rig" and you would like to combine it with a complete rig like rigify (which includes its own deform rig, controllers rig, mechanical rig, ...)?

Sadly it isn't possible. Not in an automated way I mean. You can't mix two different rigs and expect them to work together, they don't know how to "communicate" with each other.

But you have different options to choose from:


Start from the original deform rig and manually build your own controls and mechanics around.
Discard the original deform rig and create a whole new one with rigify.
Try to rename the meshes's vertex groups to the corresponding rigify's deform bones so that the rigify rig uses the original vertex groups for deformation, and make some adaptations when needed.


There isn't one best solution IMHO, they each have their pros and cons. But depending on your situation and goals, some might be more adapted.
If you just want to have an animation ready rig as soon as possible and you feel comfortable with retargeting weights, then the third option looks fine.
If you want your rig asap but don't quite enjoy the tricks of retargeting, then the second option looks more appropriate.
If you want to learn how to rig and skin, then the first option is hand on the best. Or even start from scratch, it's even more instructive.
If you know how to rig, then... Well, whatever, really.